Hydrolysis-condensation products of trialkoxysilanes, such as of methyltrialkoxysilanes, are of commercial interest, because such products possess several properties unique among organopolysiloxanes. Since, for example, such products have no carbon-to-carbon bonds, they transmit well ultraviolet radiation, and of all organopolysiloxanes they contain a relatively high proportion of inorganic constituents.
However, it is extremely difficult to control the hydrolysis and condensation rates of organotrialkoxysilanes. Most silanols, that is, silicon compounds which possess one or more hydroxyl groups bonded directly to a silicon atom, tend to form the corresponding ether analogs (siloxanes) by an equilibrium reaction that may be regarded as the loss of one molecule of water per two such hydroxyl groups. The reactivity of silanols, especially silanetriols, to form siloxanes is so great that selectivity in product formation is seldom possible, and complexity of the reaction mixture often precludes identification of specific products.
The high activity of most silanols makes it desirable to form them in situ. This is usually accomplished by hydrolysis of precursors which contain silicon atoms attached to substituents convertible to hydroxyl groups. These hydrolysis reactions may be illustrated as EQU Y.sub.3 SiA+H.sub.2 O=Y.sub.3 SiOH+HA EQU Y.sub.2 SiA.sub.2 +2H.sub.2 O=Y.sub.2 Si(OH).sub.2 +2HA EQU YSiA.sub.3 +3H.sub.2 O=YSi(OH).sub.3 +3HA EQU SiA.sub.4 +4H.sub.2 O=Si(OH).sub.4 +4HA
in which Y represents, for example, an organic group such as hydrocarbon radical, and A represents a hydrolyzable group such as alkoxy, acyloxy, halogen, etc.
It is known that an acid medium catalyzes hydrolysis and siloxane formation in the types of reactions just described. It was previously thought that control of the reaction conditions and particularly of the acid pH condition was an absolute necessity to prevent an uncontrollable reaction, such as premature gelation. The acute sensitivity of silanetriols, such as methyl silanetriol, to acid catalysis in general precluded the use, as precursors, of methyltrichlorosilane, methyltriacetoxysilane, and similar derivitaves which liberate acid by-products during hydrolysis. As a result attention has been directed mainly to trialkoxysilanes such as methyltrialkoxysilane.
Accordingly, prior research relating to the hydrolysis-condensation of trialkoxysilanes to organopolysiloxanes has concentrated on control of an acid pH. To avoid gelation and effect hydrolysis and polysiloxane formation at a conveniently rapid and controllable rate, it was regarded as essential to maintain the acidity of the hydrolysis-condensation reaction within certain limits. The amount of acid catalyst, such as hydrochloric acid, used was previously fairly carefully regulated. A slight deviation from a prescribed amount, such as from inadvertent mistake, could cause large variations in the polymer produced. The reactions of hydrolysis and condensation are normally so fast and sensitive that they could not be easily controlled.
U.S. Pat. No. 3,389,114 to Burzynski and Martin discloses using a reaction mixture of a methyltrialkoxysilane and from 0 to about 700 parts of acid, as an upper limit, expressed as HCl, per million parts of total silane reactant material and water. This disclosure is limited to methyltrialkoxysilanes in which the alkyl group has less than four carbon atoms, since the system is regarded as even more sensitive to other alkyltrialkoxysilanes.
U.S. Pat. No. 3,428,599 to Newing teaches prevention of premature gelation of organopolysiloxanes by controlling the acidity of the polymers during a precuring step with a silazane such as hexamethyldisilazane.
U.S. Pat. No. 3,474,070 to Levene discloses a process for hydrolyzing organotrialkoxysilanes at neutral conditions using an ion-containing hydrolysis catalyst, such as a ferric alkoxide or a ferric chelate.
U.S. Pat. No. 3,479,316 to Levine describes a process for hydrolyzing trialkoxysilanes at neutral conditions using a vanadium-containing hydrolysis catalyst such as trialkylvanadate.